In certain situations, a gear wheel may have a number of missing teeth along a section of its periphery. This will generally be the case where it is required that a second gear wheel, otherwise in mesh with the first gear wheel, have the possibility of rotation independent of the first gear wheel. When the first gear wheel is turned so that the section having missing teeth faces the second gear wheel and therefore the wheels are not in mesh, the second gear wheel can be rotated without rotating the first wheel. However, if now the first gear wheel is turned so as to re-mesh with the second wheel, the individual gear teeth may not be aligned and problems can arise with re-engagement of the wheels. Significant tooth wear can result and in certain circumstances the gears may even jam together.
In one type of postage meter, a printing drum is provided in which separate value print wheels can rotate to present a selected value on the drum for printing. This rotation of the value print wheels can be achieved by longitudinal movements of corresponding value selection racks slidably mounted within a shaft on which the print drum is mounted. The whole of this shaft and print drum arrangement must be able to rotate in order to provide the required printing action of the print drun, and therefore some means has to be provided for adjusting the longitudinal position of each value selection rack with respect to the shaft. One such means includes gear wheels mounted concentrically on the shaft and movable relative to the shaft with the provision of means for translating the rotational movement of each gear wheel into an appropriate longitudinal movement of the value selection rack. This can take the form of an internal pinion and shaft in association with a helical gear, or a bevel gear and pinion gear arrangement acting on a tooth portion of the value selection rack.
Value selection in such a postage meter is thus achieved by rotating the appropriate gear wheel with respect to the print drum shaft when this shaft is in its stationary home position. This rotation can be achieved simply by providing an appropriate meshing gear train in which value setting is achieved by actual manual rotation of selector wheels attached to the gear train. Another value selection mechanism is disclosed in our co-pending European Patent Application No. 85308399.6 (our ref: E129/2), in which the appropriate gear rotation is achieved by depression of value setting push buttons.
However such rotation of the value setting gear wheels on the print drum shaft is effected, it is initially necessary for these gear wheels to be rotatable about the shaft to allow such setting, but subsequently to be fixed with respect to the shaft, once setting has been completed and a franking operation is commenced resulting in rotation of the print drum and its shaft. Without the fixed relationship between the value setting gear wheels and print drum shaft during franking, it would be possible for the selected values to be changed by relative rotation between gear wheels and shaft. Clearly this would be unacceptable.
This required function of the value setting gear wheels can be achieved by including sections of missing teeth on selected gear wheels of the mechanism, such that certain gears are not in mesh when the print drum shaft is in its stationary home (value setting) position, but are caused to mesh and thus prevent turning of the value setting gear wheels relative to the shaft once a franking operating causes the shaft to move away from its home position.
If one or more of the value decade settings has been inadvertantly moved to a position between two adjacent discrete values, a number of problems could arise.
Firstly, as described above, re-meshing of the gears (once a franking operation has commenced) may be a problem if the teeth on the gears are misaligned.
Secondly, if the value setting is at an intermediate position, the respective print wheel will also be at an intermediate position, resulting in improper franking.
Finally, the postage meter will also include some means for accounting for the franked values, and in the above-described type of postage meter, such accounting means may include "rising tooth" mechanisms adjacent the value setting gear wheels on the print drum shaft. These mechanisms will be described in greater detail hereinafter, but briefly the number of teeth to have risen is dependent on the value set, and these teeth are then counted mechanically when the print drum shaft rotates during franking and are added to a cumulative total in a counter. Intermediate setting will result in uncertainty whether or not a particular unit in a value decade has been registered by the counter.
In has previously been proposed, for example in UK Patent Specification No. 1 584 391, that gear teeth be provided with a degree of resilience in a direction tangential to the gear periphery. In contrast to the present invention, this proposal was directed to ensuring more even meshing between gears, and equalising the forces acting on the individual gear teeth.